The present invention relates to a method of manufacturing a lithographic printing plate by electrophotographic process.
In conventional lithographic printing, a plate coated with a photosensitive resin is mainly used. This resin is sensitive to ultraviolet rays (of less than 450 nm). This sensitivity is extremely low as compared with that of silver halide photosensitive material, or photoconductive materials now used for electrophotography. Therefore, in conventional photographic printing plate making system, a negative original film is firstly prepared with a silver halide lith film, and then the original film is contacted on a lithographic printing plate in a vacuum printer. They are exposed by the ultraviolet rays and the lithographic printing plate can be prepared by this contact print process.
Recently, a new plate making method which is claimed to be the printing plate making process with the camera exposure speed has been developed, in which the using of the photographic lith film is excluded for the purpose of saving labor and resources, and preventing environmental pollution.
Several methods of making a printing plate by electrophotographic process are already known, and many proposals have been made for its use to form the lithographic printing plate. For instance, there is known an electrophotographic method utilizing a zinc oxide photoconductive layer. According to this method, a toner image is formed on a zinc oxide photoconductive layer coated on a sheet of paper or aluminum; the area of the toner image has oleophilic properties; and the zinc oxide photoconductive layer of the non-image area is treated to have hydrophilic properties, thereby providing a lithographic printing plate. The plate thus obtained meets the requirements of cost, speed of making and environmental conservation, but it is far inferior in durability to the pre-sensitized plate (PS plate) utilizing the conventional photosensitive resin, Accordingly, it is useful only for shortrun printing in what is called "light printing" or "office printing".
Lately commercial printing and newspaper printing by electrophotographic process has been announced, such as the "PYROFAX" system of 3M Co. (Minnesota Mining and Manufacturing Co.) or "ELFASOL" system of Kalle AG. 3M's "PYROFAX" is so-called "a magne-dry system" with inferior resolution due to coarse grains of toner and is low durability and complicated handling. Kalle's "ELFASOL" is non-transfer type imaging system using an aluminum plate coated with an organic photoconductor which has the merit of no deterioration of toner image during transfer, but has the defects of requiring the photoconductive layer to be considerably over 5.mu. thick to hold the electric charge for having both properties of electrophotographic performance and mechanic strength in printing.
Many other methods of making a printing plate by electrophotographic process have been proposed. For instance, U.S. Pat. Nos. 3,071,070 and 3,063,859 disclose a method of transferring a toner image directly onto a metal plate for printing to make a lithographic printing plate. In these methods the transfer effect is poor quality with a so-called "hollow" image, and the image reproducibility is unsatisfactory, because of direct transfer onto an electroconductive base plate. The U.S. Pat. No. 3,647,499 discloses a method of transferring, with a thin layer of insulating liquid inserted between the metal plate and the photoconductive substrate. This method had the defects of a liquid being used for transfer and the toner being dispersed in the liquid. Meanwhile published Japanese patent application Nos. Sho 50-1801, 51-129302 and 51-143408 disclose double exposure methods, i.e., image exposure by electrophotographic process and total post-exposure by ultraviolet rays with use of the conventional PS plate or photosensitive relief plate for the purpose of assuring high durability; but this method is deprived of the desirable feature of electrophotographic process that requires exposure only to visible light.